The present invention generally relates to devices that sense fluid and measure volumes of fluid. More specifically, the present invention relates to sensors that measure fluid volumes, preferably medical fluids, for example, fluids used during dialysis therapy.
Due to disease, insult or other causes, a person's renal system can fail. In renal failure of any cause, there are several physiological derangements. The balance of water, minerals and the excretion of daily metabolic load is no longer possible in renal failure. During renal failure, toxic end products of nitrogen metabolism (urea, creatinine, uric acid, and others) can accumulate in blood and tissues.
Kidney failure and reduced kidney function have been treated with dialysis. Dialysis removes waste, toxins and excess water from the body that would otherwise have been removed by normal functioning kidneys. Dialysis treatment for replacement of kidney functions is critical to many people because the treatment is life saving. One who has failed kidneys could not continue to live without replacing at least the filtration functions of the kidneys. Hemodialysis and peritoneal dialysis are two types of dialysis therapies commonly used to treat loss of kidney function.
Peritoneal dialysis uses a dialysis solution or dialysate, which is infused into a patient's peritoneal cavity. The dialysate contacts the patient's peritoneal membrane in the peritoneal cavity. Waste, toxins, and excess water pass from the patient's bloodstream through the peritoneal membrane and into the dialysate. The transfer of waste, toxins, and water from the bloodstream into the dialysate occurs by diffusion and osmosis because there is an osmotic gradient across the peritoneal membrane. The spent dialysate is drained from the patient's peritoneal cavity to remove the waste, toxins and water from the patient. New dialysate replaces the spent dialysate and the process repeats.
During dialysis therapy, a dialysis fluid exchange generally includes draining spent dialysis fluid from the peritoneal cavity and filling the peritoneal cavity with fresh dialysate. Keeping track of the amounts or volumes of dialysis solution drained from and supplied to the peritoneal cavity is important for proper dialysis therapy. A typical amount of dialysate solution drained from and supplied to the peritoneal cavity of an adult during an exchange can be roughly two to three liters. Dialysis fluid exchanges have been performed manually, usually by the patient, or automatically, by an automated dialysis machine.
Manually performed dialysate exchanges require that the patient or other person performing the exchange manually drain the proper amount of fluid from the peritoneal cavity and manually supply the proper new amount of fluid to the peritoneal cavity. Errors in the amount of fluid exchanged can occur when relying on manual fluid exchanges. If it is desired to know the cumulative amount of fluid used over multiple exchanges, the cumulative total fluid must be tracked manually.
So-called continuous flow peritoneal dialysis (“CFPD”) systems that purport to provide continuous dialysate flow exist. In these systems, “spent” dialysate (waste laden dialysate) from the patient collects in a drain bag, which is discarded, or which runs into a household or other drain. A patient typically fills the peritoneal cavity with up to three liters of dialysate in one treatment. As a consequence, known CFPD dialysis treatments require a large amount of fresh dialysate, which must be tracked or calculated.
Automated dialysis machines use one or more fluid pumps to perform the dialysate exchanges, i.e., the pump pumps spent dialysate fluid out of the peritoneal cavity during the drain mode and pumps dialysate into the cavity during the fill mode. Known automated dialysis machines have measured and controlled the amount of fluid drained and filled during an exchange. One Automated Peritoneal Dialysis (“ADP”) system uses a reference chamber and Boyle's Law to determine the volume of fluid pumped.
Another known method of measuring the volume of fluid used during dialysis treatment is weighing the fluid with a scale. In preparation for filling of the peritoneal cavity, a bag of dialysate is placed on a scale and the weight is recorded. After fluid is removed from the bag (supplied to the peritoneal cavity) the weight of the remaining fluid in the bag is read and recorded. The volume of fluid supplied to the patient can be determined from the reduced weight of the bag of fluid. Similarly, during the draining of the patient, a weight increase of the bag of fluid is used to determine the volume of fluid removed from the peritoneal cavity. Weighing the fluid to determine fluid volume requires a scale and a bag or other reservoir to be placed on the scale.
Another known method of measuring the volume of fluid pumped by a pump is simply by knowing the volume of the pump chamber and assuming that the chamber completely fills upon each pump stroke. The volume of fluid pumped is thereby determined by counting the number of pump strokes and multiplying the number by the volume of the pump chamber.
However, it is frequently the case that the pump chamber does not completely fill with fluid. For example, air can be present in the pump chamber or the pump may simply not complete a full stroke. Accordingly, the measurement of the amount of fluid pumped may be inaccurate, for example, less fluid may be actually pumped than the amount calculated. These drawbacks can become more pronounced over many pump strokes, such as when a smaller pump requires numerous pump strokes to fill or empty the peritoneal cavity.
The accuracy of measuring fluid volume pumped can become more difficult when the size of the pump chamber is small and the pump is operated through many pump strokes. For example, certain dialysis systems must pump two to three liters of dialysate into the peritoneal cavity with a pump having an inner volume of about ten to fifteen milliliters. Numerous pump strokes, about two-hundred strokes of a completely filled pump chamber, are required to pump the two to three liters of dialysate. If the pump chamber is not precisely and consistently filled or emptied with dialysate for each pump stroke, the total amount of dialysate pumped over the total number of pump strokes may be inaccurately measured.
Accordingly a need exists for devices that can accurately measure pumped fluid volume.